It has long been known that, at least in theory, the capacity of a wireless communication system can be increased by employing more than one antenna at the transmitter, at the receiver, or at both the antenna and the receiver.
Various multiple-antenna systems have been considered. Among them is a class of systems referred to as “Multiple Input-Multiple Output” (MIMO) systems. In typical MIMO systems, the transmission of a block of message information can be distributed across an array of two or more antennas, and across two or more discrete time intervals, which are sometimes referred to as “transmit symbol intervals” or “channel uses”.
The above-described distribution across multiple antennas may be thought of as distributing the transmitted signal across “space,” and the distribution over multiple time intervals may be thought of as distributing the transmitted signal across “time.” The signal which is to be transmitted is described by a matrix which, accordingly, is referred to as a “spacetime” signal matrix.
In one common form of notation, a typical spacetime signal matrix is T×M, wherein each of the T rows represents a distinct transmit symbol interval and each of the M columns represents a distinct transmit antenna. Within each row, the entry at each of the M column positions is a complex number representing a baseband-level signal value that is to be modulated onto a carrier and transmitted from its respective antenna during the transmit signal interval corresponding to that row.
Each row of the spacetime signal matrix is referred to as a “transmit symbol vector.” Each transmit symbol vector may be though of as a row vector containing complex-valued entries. Depending on the specific coding scheme, each of these complex-valued entries may, e.g., be a scalar symbol from a symbol constellation, or a sum of such symbols.
The Alamouti spacetime code is a particular MIMO coding scheme that has received much favorable attention. The Alamouti code works best when the signals from the respective transmit antennas are uncorrelated.
Another way to take advantage of multiple-antenna transmission is by the well-known method of beamforming. Beamforming works best when there is strong correlation among the signals from the respective transmit antennas.
There remain opportunities to improve system performance in the intermediate regime in which there is a moderate amount of correlation among the transmission antennas of a MIMO system.